The present invention relates to a wireless communication system. More particularly, the present invention relates to a wireless communication system in which a plurality of wireless terminals communicates with each other through a fixed-length communication frame periodically generated by a base station and also relates to a wireless terminal device which is used in such a wireless communication system.
A wireless LAN (Local Area Network) is a communication system in which wireless terminals connected to the LAN make the transmission and reception of information in units of one packet or frame. In the wireless LAN, a part of a transmission medium such as coaxial cable or optical fiber used in a wire LAN is replaced by a wireless channel.
The construction of a communication frame used in the wire LAN has been disclosed in detail by, for example, ISO 8802-3, "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical specifications" which is the international standard of ISO.
This communication frame is composed of a header portion and an information data portion. The header portion includes a preamble for establishing bit synchronization, a frame flag indicating the head of a frame, a destination address, a source address, and a plurality of fields for indicating the length of data and so forth. Each terminal device analyzes the destination address in a received communication frame to judge whether or not the communication frame is one destined for that terminal itself.
One example of systems, in which a function similar to that of the wire LAN is realized and a part of its communication channel is replaced by a wireless channel, has been disclosed by the article "Wireless In-Building Network Architecture and Protocols", IEEE Network, Vol. 5, No. 6, November 1991 (hereinafter referred to as reference 1).
In a network system disclosed by the reference 1, communication data between wireless terminals is all transmitted through a base station. Access requirements from wireless terminals are scheduled by the base station, and each wireless terminal having data to be sent (or each sending terminal) transmits the data in accordance from an indication from the base station. The base station receives data from a wireless terminal and sends the received data to a receiving terminal designated by a destination address.
A communication frame used in the network system disclosed by the reference 1 is defined to include a first control information field (or time slots) for allowing each sending terminal to send an access request, a data field (or time slots) for allowing an access-granted sending terminal to send a transmission message, and a second control information field (or time slots) for allowing the base station to send information indicative of the grant/no-grant of access, in the mentioned order.
In the wire LAN, it is general that the length of data capable of being sent in the date field of each communication frame is made not shorter than one kilobyte. On the other hand, in the wireless LAN having a high bit error rate in a wireless channel, it is general that each terminal divides a transmission message into a plurality of fragments with relatively short length and sends the message in units of one fragment in order to improve the total transmission efficiency for which the generation of data retransmission is taken into consideration. In the network system of the reference 1, too, the transmission message is sent in a form divided into a plurality of fragments with fixed length. In order to allow a receiving terminal to receive the message in units of one fragment, each fragment is provided with the same address information as address information (or source address and destination address) included in the transmission message. Also, each fragment includes control information for message reassembling and the receiving terminal reassembles the received fragment into the original message on the basis of this control information.
As a wireless channel access system allowing each terminal to control individually data transmission is known, for example, a CSMA (Carrier Sense Multiple Access) system disclosed on pages 208 to 212 of "Packet Switching Technologies and Applications Thereof" published by the Institute of Electronics and Communication Engineers of Japan (hereinafter referred to as reference 2). In the CSMA, each terminal having transmission data detects a carrier at the time of data transmission to check whether or not a channel is used by another user and initiates the transmission of the data after it has been acknowledged that the channel is vacant.
In the wireless LAN system, since cable wiring between a terminal and a base station becomes unnecessary, data communication by a handy terminal using LAN becomes possible. In this case, the realization of a wireless transceiver with small power consumption taken the operation of a buttery into consideration becomes an important problem for the purpose of implementation of small size and light weight of the handy terminal.
In the LAN system, data from another terminal arrives randomly in contrast with a synchronous communication system, for example, a telephone network. Accordingly, in order to allow each wireless terminal to always receive data which arrives from another terminal without notice, that wireless terminal has a need of keeping the power source of its wireless transceiver always in an ON condition.
For example, in the system of the reference 1, unless each wireless terminal reads the destination address of each fragment header in a communication frame, that terminal cannot know whether or not the fragment is destined for that terminal itself. Further, in order to identify the position of the fragment, it is necessary to make synchronization at the head of a communication frame. Therefore, each terminal has a need of keeping its wireless receiver always in an operating condition, thereby repeating the analysis of received data.
In the system of the reference 2, too, each terminal has a need of keeping the power source of the wireless receiver always in an ON condition since data destined for that terminal itself is randomly generated. Namely, there is a problem that the electric power is consumed in a period of time when no available data is received.